Sports · Part Two · Assessment and Movement
Lesson 11 / 64
Clinical Biomechanics
Every movement an athlete makes is force resolved through bone, and biomechanics is how we read it.
Clinical biomechanics is the study of the forces an athlete produces and absorbs, split into kinematics for the shape of motion and kinetics for the force behind it. Force does not stay where it lands: myofascial transmission carries it between all muscles in a limb segment. The Unified Model of Tone reads the body as a pre-tensioned structure rather than a row of dominoes.
Sprinter moment arm
25 percent smaller
Fascicle to lever ratio
50 percent larger
Mass meeting the ground
0.08 of body mass
Force transmission
Between all limb muscles
Kinetics and kinematics.
Kinematics describes motion without asking its cause: speed, arc, height. Kinetics describes the forces that produced, stopped or changed that motion, measured in newtons.
Myofascial force transmission.
Force passing from a muscle into neighboring structures through connective tissue rather than only along its own tendon. It occurs between all muscles within a limb segment.
01What the measurements show
The Numbers Behind Clinical Biomechanics
Eight findings on how force actually travels through an athlete.
02The two lenses
Motion and Its Cause Are Separate Questions
Clinical biomechanics splits every movement into two questions. What did the motion look like, and what force created it? Kinematics describes the motion itself: how fast a limb travels, how high it rises, and through what arc, without asking why. Kinetics asks the harder question, which internal and external forces produced, stopped, or changed that motion.
For an athlete this distinction is the whole game. A sprinter and a coach can both see a stride. Only the kinetic view reveals the ground reaction force driving it and the muscle tension steering it.
Gravity, friction, external resistance, and muscle tension all act at once. Reading them apart is how we find the link between what an athlete does and what their body is asking of itself.
03The first law
Force Equals Mass Times Acceleration, and Then It Has to Go Somewhere
Force is a push or a pull, and in the body it is the cause of every acceleration an athlete generates. The relationship is exact. Force equals mass times acceleration, so a heavier athlete needs more force to reach the same speed, and a sharper change of direction demands a sharper spike of force. Force is a vector, carrying magnitude and direction, measured in newtons.
Force does three things to a body. It translates the body through space, it rotates the body around an axis, and it deforms tissue under load. Sport lives in all three. A lineman translates an opponent backward, a pitcher rotates a torso into a throw, and a landing knee deforms cartilage and ligament with each impact.
Where the impact actually lands
One measurement makes the scale concrete. Running ground reaction force patterns can be predicted by treating the body as just two moving masses: the contacting lower limb at 0.08 of body mass, and the remaining 0.92 Udofa 2019. The whole athlete does not strike the ground. A small fraction does, and the rest arrives afterward.
When we assess an athlete we are tracing where these forces concentrate, because force that gathers in the wrong tissue is how injury begins. Knowing the load a given exercise places on a tendon is harder than it sounds. A scoping review of in vivo estimation methods found that the load acting on the tendon during loading programs is usually unknown Escriche-Escuder 2022.
04The levers
Bones Are Machines, and the Right Machine Depends on the Event
Every moveable bone in the body is one arm of a lever, and the athlete who moves well exploits that machinery without thinking. A lever has three parts. A fulcrum, which is the joint axis. A force arm, from the axis to where muscle pulls. A resistance arm, from the axis to the load. The ratio between those arms sets the trade between speed and strength.
The body runs all three lever classes. A class one lever places the fulcrum between effort and load, like the head nodding on the atlas. A class two lever puts the load between, as the calf raises the body at the ball of the foot. A class three lever, the most common in sport, puts muscle force between axis and load, like the biceps curling the forearm. It trades raw force for speed and range at the hand.
The task decides which Achilles moment arm length helps
Measure one lever across the literature and something instructive happens. Collegiate sprinters carry Achilles tendon moment arms 25 percent smaller than height matched non-sprinters, with fascicles 11 percent longer and a fascicle to moment arm ratio 50 percent larger Lee 2009. A short lever and long fascicles suit explosive push-off.
Now change the event. In competitive marathon runners, a longer Achilles moment arm was associated with better running economy Kovacs 2021. The arrangement that helps a sprinter is not the arrangement that helps a marathoner.
Then narrow the group. Eighteen highly trained 100 meter sprinters were separated into fast and slow by personal best, 10.30 seconds against 10.70. Achilles moment arm and lower leg geometry showed no significant correlation with sprint performance at all Karamanidis 2011.
Read together the three studies report one result about levers. Geometry sets what is mechanically easy, the task decides which geometry helps, and inside a group of athletes who have already been selected for that geometry it stops separating anyone. The lever is real and it is not the answer by itself.
The lever also refuses to hold still. Simulating a knee with 12 degrees of freedom showed the knee extension moment arm varying through walking and running rather than sitting at a fixed value Wheatley 2021. A lever arm measured on a table does not describe what happens at speed.
05Reading gait
Running Is the Screen That Exposes the Pattern
Gait analysis is the most revealing biomechanical screen in sport, because running exposes every asymmetry an athlete carries. The gait cycle divides into stance, when the foot loads the ground, and swing, as the limb advances. At running speed stance shortens and ground reaction force climbs onto a single limb.
Support the body and those forces fall. Running on a lower body positive pressure treadmill with body weight support reduced vertical ground reaction force alongside changes in stride characteristics Raffalt 2013. The load an athlete absorbs is adjustable, which is the whole basis of graded return to running.
What we are actually reading
Inside that cycle we read the chain. A hip that drops at midstance, a knee that collapses inward, a foot that overpronates: each one shifts force up the kinetic chain to a tissue that was never built to carry it. The nervous system writes this pattern, since gait is a rhythm generated and refined by spinal and cerebellar circuits.
Read the pattern accurately and you find the cause, not just the sore spot the athlete points to.
06Not a domino line
A Local Change in Tension Does Not Stay Local
The kinetic chain is usually taught as a row of dominoes. Force enters at the foot, passes link to link, and a weak link breaks the transfer. That picture is useful and it is incomplete, and the experimental work says why.
Force does not travel only along tendons. Reviewing intramuscular, extramuscular and intermuscular pathways, the conclusion is that myofascial force transmission occurs between all muscles within a limb segment Huijing 2007. Force generated within the sarcomeres of an antagonist may then be exerted at the tendon of a target muscle or its synergists.
Read that twice. A muscle pulling the other way can deliver its force to the tendon of the muscle it opposes. No domino model predicts that.
The tissue that carries it
The pathway is the connective tissue itself. Endomysium transmits contractile force between adjacent fibers within a fascicle, and perimysium and epimysium can serve as force transmission pathways Purslow 2010. Perimysium also defines slip planes that let fascicles shear past each other.
The Unified Model of Tone reads this as tensegrity rather than plumbing. A local change in tension does not stay local. When one corner of a pre-tensioned structure is stressed, the whole structure adapts to preserve balance, which is why remote effects that a purely local view finds puzzling keep showing up in clinical practice.
It is also why a biomechanical fault is so often a control fault wearing a mechanical mask. A knee that caves is frequently a hip that fires late.
07What we corrected
One Claim Softened by the Evidence
The hip weakness story is more popular than the data support, and this page repeats it in a narrower form now. A systematic review of hip abduction strength and running injury screened 1,841 articles and included 11 Mucha 2017. Meaningful differences appeared for iliotibial band syndrome, where three of five studies found weakness in injured runners and both of the methodologically strongest found a relationship.
For patellofemoral pain, medial tibial stress syndrome, tibial stress fracture and Achilles tendinopathy, the results did not form associative or predictive relationships. The authors are explicit that the association is unclear as a significant factor.
So the caving knee remains worth examining, and hip abduction weakness is not a general explanation for lower limb running injury. The page also carried a quotation attributed to Dr. Jason Dulberg that was not drawn from anything he said or wrote. It has been removed.
08The model's claim
Mechanics Describe the Movement, but Neurology Authors It
Two layers run through this page. The established science is the myofascial transmission work, the moment arm measurements, the two mass account of ground reaction force, and the hip abduction review. Those measurements belong to the biomechanists who made them.
The Unified Model of Tone puts a nervous system upstream of all of it. Biomechanics is downstream of the nervous system, because no joint moves until a motor command tells it to. The cleanest force vector and the most efficient lever still depend on timing, on which muscle fires when and how hard. That sequencing is set by the motor cortex, the cerebellum, and the spinal reflex loops that tune every contraction.
The prediction this page makes
The model treats a movement fault as a tone reading rather than a shape. It predicts that the mechanical variables clinicians measure will separate athletes poorly on their own, as the moment arm data did inside an elite group. Paired with the regulatory readouts, they should separate them well. This is a claim about how movement is organized rather than a claim about what treatment does.
The test is available with equipment sports science already owns. Record peak ground reaction force, hip adduction angle at midstance, muscle onset latency across the chain, and time to return to baseline after a standardized load test, in the same athletes across a season.
If peak ground reaction force, hip adduction angle, muscle onset latency and time to return to baseline are shown to move together within the same athletes, the unification claim is confirmed.
We assess the mechanics precisely so we can correct the signal beneath them, because an athlete who moves with clean biomechanics is an athlete whose brain and body are speaking the same language at speed.
09The tone reading
Force Is Distributed, and So Is Its Regulation
Three signatures of tone appear in the mechanics on this page.
Coupling
Force made in one muscle reaches the tendon of another across the limb segment. Coupling is the mechanism, not a metaphor.
Constraint
A moment arm 25 percent smaller sets what is mechanically easy. It constrains the athlete without deciding the outcome.
Load
Only 0.08 of body mass meets the ground at impact. The rest arrives through a structure that has to organize it.
The rest of the library carries the same logic through its other foundations. Gain is the setting that decides how much drive reaches a muscle, and prediction is the feedforward model that fires the hip before the foot lands. Time-course governs how quickly tissue adapts to the loads above, while oscillation is the rhythm gait is generated from. Set-point and input-quality describe the state a training load meets and the character of the signal that meets it. The full framework is set out in the Unified Model of Tone.
10Where this sits
How This Page Relates to the Rest of the Library
Seven places this argument continues, each with the claim that earns the link.
Takes the tensegrity reading above and works it through the whole chain rather than one segment.
Owns stride-interval variability and loading rate, the running measurements this page only frames.
Shows the same test changing its answer with the athlete guarding, which is mechanics meeting state.
Carries regional interdependence and the Panjabi subsystems in full.
Where the stretch-shortening cycle turns these lever mechanics into usable power.
The evidence that force is delivered by neural drive rather than stored in tissue.
Why a side to side difference is not automatically a fault to be corrected.
11Questions athletes ask
Questions Athletes Ask
What is clinical biomechanics and why does it matter for an athlete?
Clinical biomechanics reads the forces you produce and absorb. It splits every movement into kinematics, the shape of the motion, and kinetics, the force behind it. It matters because force that concentrates in the wrong tissue is how injury starts. The practical work is tracing where ground reaction force, lever mechanics and muscle timing route load, then addressing the motor control that authors the pattern rather than only the tissue that hurts. Mechanics tell you where the load went; they do not tell you who sent it there.
Does force really travel between muscles, or just along tendons?
Between muscles, and this is better established than most athletes realize. Reviewing intramuscular, extramuscular and intermuscular pathways, the conclusion is that myofascial force transmission occurs between all muscles within a limb segment. Force generated in an antagonist can then be exerted at the tendon of a target muscle. The connective tissue is a load path rather than packaging, and that is why a local change in tension does not stay local anywhere in sport.
Is there an ideal body structure for speed?
There is a structure that suits sprinting, and it is not the one that suits distance. Collegiate sprinters carry Achilles moment arms 25 percent smaller than non-sprinters, with fascicles 11 percent longer. In marathon runners a longer moment arm goes with better running economy. Among 18 elite 100 meter sprinters, moment arm predicted nothing about performance at all. Geometry sets what is easy; it does not decide the athlete. Inside a group already selected for that geometry, the variable stops separating anyone at all.
My knee caves when I land. Is that a weak hip?
Sometimes, and less often than the popular version claims. A systematic review of hip abduction strength in distance runners found meaningful differences only for iliotibial band syndrome. For patellofemoral pain, medial tibial stress syndrome, tibial stress fracture and Achilles tendinopathy, no associative or predictive relationship emerged. A caving knee is worth examining closely, but it is frequently a timing problem: a hip that fires late rather than one too weak. Strength testing alone will miss that, because the muscle can be strong and still arrive late.
How much force actually goes through my body when I run?
More than bodyweight, and it does not arrive all at once. Running ground reaction force patterns are well predicted by treating the body as two masses: the contacting lower limb at 0.08 of body mass, and the remaining 0.92 that arrives afterward. Supporting bodyweight on a positive pressure treadmill lowers those forces measurably, which is precisely why graded return to running works as a dosing strategy rather than a rest strategy. The load is a dial, not a switch.
What happens in a biomechanical assessment here?
Motion and the forces behind it get measured, and then the neural layer that authors them gets tested. That means gait analysis, joint position and proprioception testing, and a read of muscle firing timing and sequence across the chain. Because a caving knee is often a hip that fires late, the mechanics are mapped precisely so the signal beneath them can be addressed. The assessment is drug free and fully anti-doping compliant, which matters for anyone competing under testing. Nothing here is a diagnosis on its own; it is a map of how you are organizing force.
Can biomechanics explain why an old injury shows up somewhere else?
It can, once you drop the domino picture. Force transmits through fascia between muscles across a limb segment, so tension changed in one place is redistributed rather than contained. The Unified Model of Tone reads the body as a pre-tensioned structure in which stressing one corner makes the whole adapt to preserve balance. That is why remote effects a purely local view finds puzzling keep appearing in clinical practice. The prediction is testable: change tension in one segment and the redistribution should be measurable in another.
12The sources
References
10 primary sources, each linked to its record. Figures quoted on this page were checked against the published abstract.
Related evidence